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Show 114 MR. A. H. GA.RROD ON THE MUSCLES OF BIRDS. [Feb. 3, two latter families with the two former, or with one another. In fact the Accipitres, as generally defined, are not a natural group at all; and the Cathartida are not the least more nearly related to the Fulturida than to the Falconida. Respecting families it may therefore be said that myological peculiarities which do not involve more than a single structural change from the typical arrangement of the family are frequent, and that further differences indicate a more distinct relationship. The various opinions held by different ornithologists as to the correct division of the Orders of the Class Aves are so numerous that they make it impossible in this stage of the inquiry to discuss the myological features which they present. An inspection of Plate XVII. is sufficient to show that the formula of a bird is not of direct value in estimating ordinal characters. Looking at the whole subject from another point of view, it may now be asked, What does the arrangement in the muscles above described teach as to the major divisions of the Class Aves ? The remainder of this communication will be an attempt to answer this question. A mere glance at Plate XVII. is sufficient to show that the facts disclosed by a study of the myology of birds do not, without extraneous assistance, place the different families in their true relationship to one another. Because the same muscles are present in two families of birds, it cannot therefore be said that their kinship is extremely close, or the reverse; if such were the case we should have to put the Ardeidce with the Passeres, and to separate the Auks from the Gulls, both of which results would be strongly in opposition to the teaching of osteology. It is therefore necessary to look around to find, if possible, myological characters which have some definite relations to equally well-marked pterylographic, visceral, or osteological peculiarities. Before going further it will be necessary to clearly understand a principle which is of much assistance in working out the details of classification from a large number of unarranged facts. It is this : when any certain structure is found to exist in an unmodified form in several clearly separable members of any well-marked larger division of the Animal Kingdom, that structure must be considered typical of the division ; in other words, that structure, or the potentiality for producing it, must have existed in the common ancestor of the division under consideration ; and those of its members who are wanting in the particular structure are so because they have lost it in process of time, not because the others have separately acquired it; for the probability, if it were only a matter of probability, is very little that several distinct and different species should separately acquire a single identical structure ; whilst it is infinitely more likely that several distinct species should all lose a common character. That all Mammalia should acquire branched horns is improbable; but that many which possess branched horns should have them broken off whilst rushing through a wood, whatever species they belong to, is much more to be expected. Employing this argument with regard to the facts under discussion, |